I have a poorly-formatted book from the Kobo Store. I fixed up my personal copy of the ePub.
However, in the past I recall having some sort of issue when I tried to overwrite the Kobo book with a sideloaded copy. It's been a few years so the details are fuzzy but I remember ending up with multiple copies of the book on the device, the book disappearing entirely when I tried to remove the phantom copies, then having to repair account to get it back. Also weird sync issues.
Has anybody else done this recently? If so, how does it work?